Garden Projects · 9 min read · Intermediate

Decking Basics: Planning, Joists, Board Gaps & Fixing

Plan a straightforward garden deck while keeping joist spacing, board gaps, ventilation, drainage and manufacturer instructions in view.

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Key points & steps

  1. Decide whether the deck is ground-level, raised or attached to a building, because height, structure, guarding, access and planning/building requirements can change substantially between these arrangements.
  2. Check the ground, drainage and finished level before building the frame. Avoid creating a deck that traps water against the house, blocks air bricks or raises the finished surface inappropriately near damp-proof courses.
  3. Use timber grades, supports and structural details suitable for the span and loading. Get Your Project Right’s material estimate does not design a load-bearing deck frame.
  4. Follow the chosen board manufacturer’s maximum joist spacing. Different timber and composite board profiles can require different centres, and angled board layouts often need closer support.
  5. Plan board direction, perimeter cuts and any picture-frame borders before fixing the first board so joists and noggins are positioned where fixings will actually be needed.
  6. Maintain the board gap specified by the manufacturer. Timber needs room for moisture movement and drainage, while composite products have their own side and end-gap requirements.
  7. Provide the ventilation/clearance required beneath the deck and around walls. Do not seal the deck into a damp, unventilated void.
  8. Use corrosion-resistant fixings specified for the board and framing system and keep fixings the required distance from board ends/edges to reduce splitting or damage.
  9. Inspect the completed deck for proud screws, unsupported edges, trapped water and trip hazards before use.

Practical tips

  • Do not copy one generic joist-spacing figure onto every deck; product span tables are more reliable.
  • If you plan a hot tub, heavy masonry planter, balcony-height deck or significant raised structure, get structural advice rather than treating it as a simple DIY platform.
  • Composite boards can expand lengthwise with temperature, so end gaps and fixing systems matter as much as side gaps.

What you may need

  • Decking boards and manufacturer instructions
  • Suitable structural timber/support system
  • Corrosion-resistant fixings
  • Spacers
  • Saw and drill
  • Level/string line
  • PPE

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